Attention Deficit Hyperactivity Disorder (ADHD) is the commonest psychiatric diagnosis in American children. Its etiology is unknown. We and others have shown that lead at low doses affects attention in children as measured by structured behavioral inventories e.g., reaction time under varying intervals of delay; and by teachers questionnaires. Behavioral and neurochemical studies in rodents and primates given lead support these findings. Whether lead is among the causes of clinical ADHD has not been investigated. This is a case control study of 250 ADHD subjects diagnosed at the Western Psychiatric Institute and Clinic and 250 appropriate controls. Hypotheses to be tested are that lead increases the risk for ADHD, and that lead exposure is associated with increased alcohol and substance use. A comprehensive assessment battery on the ADHD on the ADHD subjects evaluating behavior, cognition, use of alcohol and substances, health and socioeconomic factors has been collected. The comparison sample will be matched in range to the ADHD sample for the following variables: age within one year, gender, ethnicity, single versus two parent household, and parent education. Past history of lead exposure will be measured by K-like x-ray fluorescence, a technique that we have been using for 5 years. ADHD subjects and controls will be evaluated first by Mantel-Haenzel procedures for stratified data, and then by logistical regression, controlling for appropriate co-variates: age. sex, race, family status (e.g., two parents living in home, divorced, never married), family history of ADHD and other psychiatric disorders (SCID), parent education, socioeconomic status, family size, family alcohol use, and family conflict (Conflict Tactic Scale and Dyadic Adjustment Scale). Use of alcohol and substances will be shown to increase the risk for clinical ADHD, an estimation between lead and treatment outcome will be evaluated. If lead is shown to increase the risk for clinical ADHD, an estimation of the proportion of this common problem attributable to a lead would be identified, and an opportunity for authentic prevention of this segment made available.